1. Field of the Invention
The present invention relates to a surface acoustic wave apparatus used in a mobile communications device, such as a mobile phone, a device mounted to a vehicle, a device for medical use, etc., and a communications device using the same, and more particularly to a surface acoustic wave apparatus and a communications device characterized by its structure such that prevents a discharge breakdown in an IDT (Inter Digital Transducer) electrode.
2. Description of the Related Art
A surface acoustic wave apparatus, such as a surface acoustic wave resonator and a surface acoustic wave filter, has been widely used in wireless communications devices of various kinds using a microwave band, devices mounted to a vehicle, devices for medical use, etc.
A conventional surface acoustic wave apparatus has a problem that a discharge breakdown occurs in an IDT electrode when exposed to an abrupt change in temperature due to the pyroelectric property of the piezoelectric substrate, which results in deterioration of the electric characteristic.
Hence, in order to prevent a discharge breakdown in the IDT electrode, improvements have been added to date.
For example, a structure in which paired comb-teeth-shaped electrodes forming the IDT electrode are connected to each other via a thin-film resistor has been known as the conventional surface acoustic wave apparatus for which the discharge breakdown preventive measure is taken.
Ta—SiO2 or Nb—SiO2 is used as a material of the thin-film resistor that electrostatically short-circuits the paired comb-teeth-shaped electrodes forming the IDT electrode.
Charges induced on the IDT electrode are forced to flow through the thin-film resistor, so that the thin-film resistor can prevent the occurrence of a discharge breakdown between the opposing comb-teeth-shaped electrodes.
The conventional surface acoustic wave apparatus described above, however, has the problems as follows.
When the resistor is connected, the electric characteristic of the surface acoustic wave apparatus readily deteriorates. In order to prevent such deterioration, it is necessary to keep a current flowing through the resistor during a normal operation small. It is therefore preferable that the resistor has a reasonably large resistance value.
The resistivity of the aforementioned material of Ta—SiO2 or Nb—SiO2 is as small as 200 mΩ·cm or less. Hence, in order to achieve a large resistance value, a sufficient length is necessary. For example, in order to achieve a resistance value of 1 MΩ, given 0.5 μm and 100 μm as the thickness and the width of the resistor, respectively, then a length as long as 25 mm is necessary.
The surface acoustic wave element employed in today's surface acoustic wave filter or the like used in a GHz band is of a size of approximately 1 mm×1 mm in length and width. Thus, an approximately 25-fold area is necessary in order to form the aforementioned resistor, which markedly increases the surface acoustic wave element in size.
By lessening the thickness and the width to the least possible level in order to reduce the resistor in size, for example, by lessening the thickness and the width to 0.5 μm and 1 μm, respectively, the length is shortened to 250 μm. It is thus possible to achieve a reduction in size. However, such an elongate resistor readily breaks due to a rise in temperature when a current flows and the resistor generates heats. When the resistor is heated to approximately 300° C. at the time of reflow soldering and a large current flows through the resistor as charges are induced by the pyroelectric effect, the resistor generates heats and the temperature rises further, which increases the possibility of a break in the resistor per se. Once the resistor breaks, there is no means for releasing the charges induced by the pyroelectric effect. It is thus no longer possible to prevent the occurrence of a discharge breakdown in the IDT electrode caused by a change in temperature.
Also, the material per se of the thin-film resistor has a problem in its heat resistance, and oxidation proceeds rapidly when the temperature rises, which raises a concern with respect to the reliability.
The invention therefore has an advantage to provide a compact, highly reliable surface acoustic wave apparatus.